1. Field Of The Invention
The present invention relates to the measurement of solid particle consistency in an aqueous flow stream. More particularly, the present invention relates to the measurement wood fiber consistency in a pulp stock flow stream.
2. Description Of The Prior Art
In many industrial processes, a finely divided particalate solid will be suspended in an aqueous medium as a slurried mixture for transport through a multiplicity of process steps. Often, it is necessary to know, to a high degree of accuracy, the solid-to-liquid ratio of such mixtures. This ratio, or some mathematical permutation thereof, is characterized by the wood pulp and papermaking industry as consistency.
The direct measurement of consistency is presently accomplished by numerous techniques depending on the particular application. A low consistency of 0.20 to 1.0 percent has been measured optically by sensing the magnitude of a particular frequency light from a constant energy emissive source that penetrates the slurry flow stream between two transparent windows in a carrier conduit. U.S. Pat. No. 3,322,960 describes apparatus having utility for this purpose.
Pulp slurry consistencies in the range of 0.50 to 5.0 percent may also be deduced from the measurement of torque required to drive, at a constant rotational speed, an axially rotating rod immersed in the slurry, said rod having whisker wires projecting normally therefrom. U.S. Pat. No. 3,488,995 describes apparatus of this type.
In the higher consistency ranges, reliable consistency measurement may be deduced from the total slurry mass shielding of beta ray transmissivity relative to that caused by the pure fluid carrier. Similarly, high consistency measurement may be obtained from the degree of electrical conductivity between two electrodes penetrating the mixture.
When the subject of measurement is a slurry of wood pulp, coagulative properties of the pulp create particular problems with such prior art techniques of low consistency measurement. Because of the low light transmissivity of a 0.50 percent slurry, windowed instrument conduits must be narrow and connected for parallel flow with the main process flow stream. Accordingly, unrepresentative samples are presented to the measured light path. Furthermore, because of the relatively small dimensions, the sample flow stream tends to clog with flocs of pulp.
Similarly, because of pulp adherence to a rotating rod and wires projecting therefrom, erroneous indications of consistency will be given by this technique.
It is, therefore, an object of the present invention to measure a low consistency wood pulp slurry by means invulnerable to prior art error sources.
It is also an object of the present invention to photometrically measure a low consistency slurry within a large diameter, main flow line carrier of such slurry.
Another object of the present invention is to measure a low consistency slurry without need of passing a light beam completely through the slurry flow stream.